Electron beam parametric amplifier with positive ion elimination



R. ADLER ET AL ELECTRON BEAM PARAMETRI@ AMPLIFIER WITH POSITIVE ION ELIMINATION Filed Jan. "7, 1964 Sept. 6, 1966 m wm Nm om NN N Hm mm Q .v Om

United States Patent O 3,271,731 ELECTRON BEAM PARAMETRIC AMPLIFIER WITH POSITIVE ION ELIMINATION Robert Adler, Northfield, Ill., and Charles B. Crumly, Palo Alto, Leigh C. Foster, Atherton, and John P. Lindley, Redwood City, Calif., assignors to Zenith Radio Corporation, Chicago, Ill., a corporation of Delaware Filed Jan. 7, 1964, Ser. No. 336,197 1f) Claims. (Cl. S30-4.7)

The present invention pertains to electron beam devices. More particularly, it has to do with transversemode electron beam parametric amplifiers.

Great success has been had with the electron beam parametric amplifier in certain applications. Such success, however, has not been without the need to overcome many obstacles both of a theoretical and of a practical nature. A demand has existed for amplifiers capable of operating at high power levels but yet having the good noise figures, stability, and phase shift characteristics of predecessor lower-power apparatus of this kind. It has been found that acceptable performance of these devices under conditions involving electron beams of high potential and current levels poses certain severe difiiculties not previously encountered with the low-power devices.

One particular difficulty arises in such devices whenever the electron velocity becomes sufficiently high to ionize any residual gas present within the tube envelope. One serious result is a variability in phase shift of the signal translated through the device. Of course, this problem suggests an effort to achieve better evacuation of the devices in order to lower the internal gas pressure and thus reduce the quantity of residual gas molecules. Hereinafter disclosed are processing techniques which result in significant improvement in the final performance characteristics of the device.

Nevertheless, a problem can still exist with respect to a small quantity of residual gas in devices operating at high beam voltages which result in electron velocities of comparatively large magnitude. Somewhat similar overall considerations previously have been encountered in velocity-modulated or longitudinal-mode traveling wave devices. For example, it has been recognized that positive ions may be trapped in a lower potential region along the electron beam of a longitudinal-mode device, known as a velocity-jump amplifier, in which the beam is projected -through sections having different potentials. Such trapped ions may degrade the noise figure of the signal carried by the space-charge waves. A solution proposed for this problem involves a different amplification mechanism in which the size of physical elements surrounding the beam is periodically made larger and smaller to vary in turn the plasma-frequency reduction factor in a periodic manner. This approach is said to enable -the electron beam to be completely enclosed within a continuous conductive shield from input to output so that, in a direction parallel to the electron beam, there are no lowerpotential areas in which the ions might be trapped.

However, the aforesaid technique is not applicable to the transverse-mode electron beam parametric amplifier which has geometrically separate sections along separate portions of the electron beam path and between which there should not be any abrupt changes of field. The differently sized sections of the aforementioned prior amplifier cause abrupt changes of voltage gradient in a direction transverse to the electron beam. Since this is the significant direction in all transverse-mode devices, such changes would cause unwanted and unstable electron motion components. Further, a collection of positive ions in any low-potential portion of the beam 3,271,731 Patented Sept. 6, 1966 ICC eventually cancels to some degree the effect of the negative space charge inside the beam, as a result of which in certain cases the ions may move outwardly and strike the adjacent electrode surfaces, thereby developing additional undesired transverse-mode noise energy.

In certain other previously known longitudinal-mode electron beam devices, electrode potentials along the beam path are purposefully arranged to create a lower potential region in which the positive ions are collected within the beam. This is done in recognition of the fact that the mutually-repelling space-charge forces inside the beam tend to cause the beam to expand in diameter. The objective of such prior devices, therefore, is to cause positive ions to be collected in the beam in an attempt to neutralize the space-charge forces. In effect, these structures achieved that which it has now been discovered should be avoided in electron beam parametric amplifiers, and especially in those of the transverse-mode electron wave type.

Accordingly, it is a general object of the present invention to provide a transverse-mode electron beam parametric amplifier which may be operated with high beam voltages while yet achieving low noise figures and good phase stability characteristics.

Another object of the present invention is to provide an electron beam device of the transverse-mode type in which different potentials may exist along the electron beam path while yet avoiding the development of undesired ion concentrations within the beam.

A further object of the present invention is to provide an electron beam parametric amplifier having its various sections spaced along the beam path but in which ion concentrations are avoided.

An electron beam device constructed in accordance with the present invention includes a substantially evacuated envelope through which an electron stream may be projected at a velocity sufiicient to ionize residual gas molecules. The stream exhibits a potential distribution across its width having a positive potential minimum at its axis tending to attract any positive ions of the gas toward that axis. Throughout the path the stream is subjected to a magnetic field which has fiux lines parallel to the path to establish a condition of transverse electron resonance. Included within the envelope is an amplifying assembly having an input coupler for imparting motion to the electrons to establish transverse-mode electron waves on the stream, a pump section downstream from the input coupler for developing a periodic inhomogeneous field having a periodicity phased with the electron motion to increase the magnitude of the waves, and an output coupler downstream from the pump section for extracting signal energy from the waves. A plurality of conductive structures included in the assembly are spaced along a path and also are individually spaced thereabout to define a drift region. The conductive structures taper apart in a given direction along the path and are biased to establish, together with the taper of the structures, a negative potential gradient along the portion of the path from one of the couplers to the other. Also included within the envelope are means spaced along the stream in the aforesaid given direction from the couplers and biased at a potential substantially negative relative to the potentials within the gradient region for collecting the positive ions; vthe gradient is sufficiently negative to draw the ions in the aforesaid direction. Finally, the envelope includes means at the downstream path end for collecting the electrons.

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best -be understood, however, by reference to the following description taken in conjunction with the accompanying drawings, in the several gures of which like reference numerals identify like elements, and in which:

FIGURE 1 is a diagrammatic longitudinal cross-sectional view of one embodiment of the present invention;

FIGURE 2 is a fragmentary enlarged cross-sectional view of an electron gun utilized in the device shown in FIGURE 1;

FIGURE 3 is a perspective view of one section of the device shown in FIGURE 1; and

FIGURE 4 is a graphical diagram useful in explaining the electrical characteristics of the device illustrated in FIGURE 1.

As shown in FIGURE 1 for the purpose of illustrating the invention, an electron beam parametric amplifier is composed of a substantially evacuated envelope 10 within which a plurality of sections are disposed along an electron beam path between an electron gun 11 and a collector 12. The principal sections of the device, stated in order of succession from gun 11 to collector 12, are a signal input coupler 13, an idl-er signal coupler 14, a twisted quadrupole pump 15, and an output coupler 16. Disposed individually between each of the successive sections are a plurality of shield electrodes 17-21, respectively. Biasing of all of the different electrodes with the required potentials is achieved by leads which feed through the envelope wall, preferably through conventonal pins in a press forming one or fboth ends of the envelope. In use, envelope is supported and positioned within an enclosing solenoid, the eld of which is depicted by arrow H, for developing a magnetic eld along the beam path with its flux lines parallel thereto.

Any of a variety of electron guns may be utilized to project a pencil-like beam through the device. Preferably, the properties of Brillouin flow are at least approximated. As is known, in such a beam the entire stream of electrons moves as a unit, like a solid pencil which rotates around the beam axis; the centripetal force applied by the flux lines of the magnetic field and tending to conne the beam diameter is made equal to the sum of the centrifugal forces resulting from the mutual electrostatic repulsion of the electrons and from their rotation with the beam as a whole. With theoretical perfection, and

if thermal motions are disregarded, there is no radial component of the electrons in the stream as they enter input coupler 13.

The particular electron gun shown in FIGURE 2 and embodied in the complete -device of FIGURE 1 is designed in accordance with the invention described and claimed in the copending and commonly assigned application of Asher Blum, Serial No. 111,831, tiled May 22, 1961, and now abandoned. Gun 11 includes an impregnated cathode 25 having an internally disposed heater coil 26 with the cathode sleeve being secured within a re-entrant double-walled cylinder 27 in turn formed inside an outer hollow cylindrical shell 2S through which pass longitudinally disposed ceramic support rods 29. A longitudinally depending skirt inside cylinder 27 serves as additional heat shielding for heater 26, the entire cathode mounting structure affording substantial heat dissipation area consistent with the high operating temperature of the dispenser-type cathode.

The overall gun construction is such that the electron beam path within it is shielded. It includes a plurality of stacked double-walled cylindrical shells, the inner walls of which define the beam-forming space. Beginning in order `away from cathode shell 28 is a ceramic -spacer annulus 30, a conductive annulus 31, another spacer annulus 32, path-defining shells 33, 34 and 3-5 individually separated by respective spacers 36 and 37, and followed by spacers 38 and 39 sandwiched about a conductive annulus 40. Finally, there is another double-walled shell 41 followed by a retaining annulus 42. The aforementioned shells and annular conductive elements are metallic, while the spacers and retainers are of ceramic material.

` ferent form.

Element 31 is apertured at its center and depressed frustoconically toward and around the end of cathode 25. Electrode 40 has an aperture on its axis which serves as part of the beam forming structure completed by the internal walls of the successively disposed shells.

In operation, electrode 31 serves as a Pierce electrode to initially confine th-e beam. A smaller-diameter aperture 43 at the entrance to -shell 33 serves to initially confine the beam diameter and this electrode also functions as an anode to draw electrons from the cathode. The beam issuing through aperture 43 expands radially outward, resulting in a sinusoidal component of radial motion having a scalloped envelope at a certain frequency of periodicity. Succeeding shells 34 and 35 serve to define a drift space for the perturbed beam, and the length of the drift space is approximately equal to an integralnumber of wave lengths at the scallop frequency. Beyond the drift space, apertured electrode 40 and shell 41 define a convergent lens having a principal plane located at a point where the perturbed beam intercepts the equilibrium radi-us of the beam within the final beam space. The strength of the lens is adjusted to apply to the beam a radial inward force which cancels the `component of radial motion developed 'by the beam as an incident of its expansion in emerging from anode aperture 43. In a typical functioning device constructed in accordance with FIGURES 1 and 2, the potentials applied to the different gun electrodes to achieve correct operation were as follows:

Electrode number: Voltage 25 O 31 -3 to -15 33 440-470 34 440-470 35 500-600 40 80-130 41 l10G-130 Within a given assembly and environment, some adjustment of these potentials will usually be necessary to remove the scallop accurately and achieve Brillouin-type ow.

Input coupler 13 (FIGURE 3) has as its principal elements a pair of deilector plates 45 and 46 disposed respectively on opposite sides of the beam path. An inductive loop 47 disposed near the internal wall of envelope 10 is connected at its ends by straps 48 to plates 45 and 46. By means of Ia similar inductive loop placed 0n the outside of envelope 10 opposite loop 47, signal energy from an input :source is delivered to coupler 13 to develop :a signal potential across plates 45 and 46. Such inductive coupling arrangements, for the couplers as well as the quadrupole section, are described in more detail and claimed in the copending and commonly assigned application of Robert Adler and Robert Cohoon, Serial No. 74,084 led December 6, 1960, now Patent No. 3,179,895. In operation, input coupler 13 responds to the input signal energy and imparts motion to the electrons as a result of which transverse-mode electron waves are established on the stream.

In the present instance, idler coupler 14 is of a dif- In construction, the active element in coupler 14 is a bi-lar helix 50. In operation, coupler 14 performs as if it were simply a longitudinally twisted version of coupler 13, the twist changing the yactual interaction frequency with the electron beam inasmuch as the electrons appear to see a iield varying at a frequency different from that of the signal energy at the output from the vcoupler. In a manner analogous to inductive loop 47 connected to coupler 13, an inductive loop (not shown) is coupled across the two windings of bi-lar helix 50. An expanded description and claims to the bi-filar helix type coupler are contained in the copending and commonly assigned lapplication of Robert Adler, Serial No. 119,931, led lune 27, 1961.

For a cyclotron wave device, as is herein embodied, bi-filar helix 50 is wound to have a pitch such that:

wr where n is the number of helix turns per unit length, n, is the number of electron resonance periods per unit length, wr is the electron resonance frequency, and w is the interaction signal frequency. The interaction frequency in this instance is that of the idler inherently produced as a result of the parametric process and the general purpose of coupler 14 is to strip noise energy from the beam at the idler frequency which otherwise would appear in the output.

Pump is a twisted quadrifilar quadrupole as fully disclosed and claimed in the copending and commonly assigned application of Glen Wade, Serial No. 289,792 filed June 20, 1963, which is a continuation of application Serial No. 747,764 filed July 10, 1958. Brieiiy, a signal from an external pump source is coupled across alternate windings of quadrifilar helix 52 by an inductive coupling arrangement analogous to loop 47 of input coupler 13. The quadrilar helix creates a field to which lthe electrons are subjected and which has a quadrupole Yent pumping field frequency is twice the cyclotron frequency of the orbiting electrons. The resulting periodic inhomogeneous field consequently has a periodicity which is phased with the electron motion in a manner which increases the magnitude of the input signal electron waves on the stream.

Output coupler 16 operates identically in principle to input coupler 13. lt is composed of a pair of deflector plates disposed individually on opposite sides of the bea-m 'and coupled to an inductive loop from which signal energy is transferred through the envelope wall to an external load. The action upon the moving electrons is the reverse of that in the input coupler.

For a more complete -description and analysis of the quadrupole amplifier, as such, reference may be made to an article entitled, The Quadrupole Amplifier, `a Low-Noise Parametric Device, which appeared on pages 1713-1723 of the October 1959 issue of the Proceed- `ings of the LRE.

lished June 19, 1963.

With reference now to FIGURE 4, the potential distribution across the beam path is of particular interest. The ordinate represents voltage and the abscissa distance from the center or axis of the electron beam; a crosssection of half the beam is depicted by the shaded :area 60. Surrounding but spaced from the electron beam is a coaxial cylindrical conductive electrode 61 which as a result of space charge in the beam carries a positive potential with respect to that of the electron beam. Curve 62 illustrates the potential distribution laterally across the beam path from a point on the inner wall of electrode 61 through the axis of the beam to a point on the opposite wall of the electrode. Between the electrode wall and the the outer edge of the beam, the potential Ve decreases in accordance with the relationship:

f a VIe-Zion). To

where V0 represents the potential depression within the beam itself, a is the radial distance from the beam axis and ro is the radius of the electron beam. The potential drop V0 within the electron beam may be calculated yfrom the expression:

wq2o2 211 (3) where wq is the plasma frequency of the electrons within the beam and fr; is their charge to mass ratio. Further,

it can be shown that the plasma frequency is expressed by the relationship:

where l is the current density Iand Vb is the potential on electrode 61. Another useful relationship is the expression:

where V0 is the average beam voltage and xq is the plasma wavelength.

' potential minimum or depression at its axis.

When the electron beam velocity in the longitudinal direction is sufiicient to ionize residual gas molecules within envelope 10, positive ions are created which drift toward the potential minimum at the beam axis. These ions have velocities very small in comparison to the electrons and which correspond to room temperature motion. When the ions are permitted to collect in la given portion of the beam path, the ion density will build up to a constant value with an upper limit at the point where the space charge would be neutralized as a result of which the beam potential no longer would be depressed. The ions then could escape radially. When such escaping ions wou-ld strike `a coupler plate, for example, noise would be induced on the electron beam which in turn would be picked up by output coupler 16.

Even though the ions do not reach a density sufiicient that they are permitted to escape radially, their presence within the electron beam tends to destroy phase stability of the amplifier. The -ions tend to accumulate in low potential regions and their effect on the beam is as if the magnetic field strength were increased; thus, the cyclotron frequency is increased. This increases the number of electron orbits through the device, resulting in a change of phase shift in the signal being amplified. VM-oreover, this phase shift is subject to instability of magnitude, being dependent upon many variables including envelope, temperature, gas pressure, magnetic field strength, and biasing potentials. Further, any radial motion of the' ions is detectable in the output coupler since the latter is a transverse-motion responsive device.

In accordance with the invention, the amplifying assembly of device 10, which includes the input and output couplers and the pump section, includes a plurality of conductive structures spaced `along the path and individually spaced about the path to define a drift region, with the structures tapering apart in a given direction along the path. As illustrated, the active electrodes of couplers 14 and 16 and of pump 15 are themselves conductive and all are tapered in the `saine direction. Still further, within the central lapertures of each of `shields 17, 18, 19 and 20, between the successive principal sections of the device, generally cylindrical drift tubes 65-68, respectively, are individually disposed. Each of these drift tubes likewise is conductive and is of frustno-conical shape so as to taper apart in the same direction as the electrodes within the couplers and the pump.

Each of the aforesaid conductive structures is biased so that, together with the taper of the conductive structures, a potential gradient is established along the beam which is negative in a direction along the path from one coupler through the other; preferably, the potenti-al decreases monotonically throughout the path portion over which the gradient extends. The dierent bias potentials are adjusted so that the gradient is sufiiciently negative to draw the positive i-ons in the direction in which the conductive structures taper apart. As is discernable from Equation 2, the taper increases the radial distance a Which in turn increases the potential difference between each adjacent electrode and the beam. As seen by the beam, this has the effect of a negative gradient along the electrode.

In the present embodiment, all of the conductive structures taper apart in the direction tow-ard collector 12, as a result of which the ions lare drawn downstream in the same direction as the electron flow. Consequently, a positive ion collector 70 is disposed within the overall collector assembly on one side of the beam path; in this instance, collector 70 is simply a U-shaped conductive element having the bottom of the U facing the beam. On the opposite side of the beam path, a similar U-shaped conductive element 71 serves as the electron collector. A transverse conductive plate 72 extends entirely across the electron beam path at its downstream end and functions together with element 71 to collect the electrons. For the purpose of deiining a 4collection chamber in which both the electrons and the ions tend to be retained, an apertured electrode 73 is disposed across the electron path immediately upstream from collector elements 70 and 71. Further cooperating in the ion collecting process is a frustro-conical cylinder 7S, tapering toward positive ion collector 70. Cylinder 75 is disposed Within the `central aperture in shield 21 between the output coupler and -collector assembly 12. In use, shield 21 and ion collector 70 are biased to a potential which is substantially negative relative to that within the region from input coupler 13 to output coupler 16.

It may be noted that, in principle, the potential gradient and the taper of the conductive structures could be reversed, as a result of which the positive ions would be drawn toward the cathode. While a suitable ion `collector could then be disposed in the immediate vicinity of the cathode, it is preferred to avoid any chance of ion penetration into the electron gun by causing the ions to travel, as shown, toward the electron collector end of device 10. Although not shown, it is also contemplated to taper the diiferent electrodes within electron gun 11, since ions can have an effect upon the focusing of the beam within the gun structure. These effects are particularly noticeable under pulsed operational conditions. However, for ordinary steady-state operation, the design and adjustment 'of the electron gun elements for proper beam formation are considerably simplified by aligning the different :gun electrodes parallel to the desired beam path.

In ope-ration, the biasing and tapering of the conducv tive structures create a potential gradient along the beam which varies the beam potential depression. This increases the speed at which the ions are swept from a region where they otherwise would tend to collect. Consequently, the ion density in that region decreases. Once an ion is caused by the potential gradient to move axially across a voltage step even as small as one or two volts, the ion attains suicient velocity to travel very rapidly through the remainder of the tube to ion collector 70. Consequently, ions coming from, for example, the region of input coupler 13 travel through the remaining sections so rapidly that their effect upon the ion density in those sections is insignificant.

As mentioned, the tapering of the electrodes serves to depress the space-charge potential along the beam in a manner which is the equivalent of an actual potential gradient along each electrode. Further, drift tubes 66-68 tend to bridge the electrode gaps which otherwise would exist between each of the principal sections. It has been found necessary to maintain a suflicient spacing between the different sections in order adequately to minimize inductive coupling therebetween. Absent the drift tubes in the shield electrodes, the radius a (Equation 2) of the effective cylinder surrounding the electron beam would become quite large, creating a potential difference which would tend to cause an ion trap to exist along the beam. Experiments have shown that gaps as small as ten to twenty thousandths of an inch with drift tube diameters of fifty to sixty thousandths of an inch are sul'licient to avoid any appreciable axial trapping of the ions, at least in combination with a stepping of the electrode potentials on the successive electrodes. In a successfully operated tube according to the invention as depicted in FIGURE l, the following biasing potentials were applied to the different conductive structures, by the connections indicated in FIGURE l by the letters A through G:

Electrode: Voltage 65 100-130 13, 66l 11G-120 50, 67 10G-105 52 70-80 68, 16 60-75 70, 75 0 71-73 400 The ranges are somewhat variable and subject to adjustment for optimum operation, with minimum beam interception by the various electrodes and With optimum phase performance. While the ranges given above the potentials applied to pump 15 and coupler 16 overlap, output coupler 16 will be adjusted to a potential to have a bias lower than that on the pump.

As will be noted from an examination of FIGURE 1, certain of the conductive structures are connected in common to a single potential source. Thus, elements 13, 66; 50, 67; and 16, 68 are commonly connected to single potential sources. This reduction in the total number of separate bias sources and the number of connecting leads which must extend through envelope 10 is another advantage stemming from the tapered structure of the different conductive elements. Where two successive electrodes have a common potential supply, the potential taper, as seen by the beam, is maintained by dirnensioning the upstream electrode to have a lesser exit diameter than the entrance diameter of the downstream one of the two.

As a further aid in preventing abrupt potential discontinuities, but in this case of radio-frequency potentials which may undesirably affect the electron flow, the input or upstream end of all four windings of quadrifilar helix 52 are connected together by a strap 53. The inductance of quadrifilar helix 52 is suflicient that strap 53 has little effect on the amplitude of the pumping field developed throughout the length of the pump section, but the pump field intensity is reduced near and at the upstream end of helix 52. Consequently, the pump iield intensity begins at a very low value and gradually increases in the downstream direction. Because of the taper of helix 52 along the stream, the pump field intensity actually then begins to decrease toward its downstream end. This change of pump field potential gradient is not harmful to the pumping action, simply causing mre gain in one portion of the pump section than in anot er.

Summarizing as to the ion-collecting arrangement, by reason of the potential gradient established from the input coupler through the output coupler, by means of both a stepping of biases on successive electrodes or electrode groups and through the inclusion of physically tapered electrode elements, regions of potential depression in an axial direction and of a magnitude sufiicient to trap positive ions are precluded. The ions are caused to travel at a comparatively high velocity along the beam path through an effective barrier established by reduced-potential drift tube 75, following which they may drift around somewhat in the collector region until finally accepted by collector 70. Further, the employment of tapered drift tubes between the different principal sections avoids the creation of potential gaps in localized regions which otherwise might cause the build-up of positive-ion concentrations. The transverse-mode electron waves, therefore, are unaffected by the generation of ions within the device.

Tubes constructed in accordance with the invention have been found to exhibit substantial gains at high power levels. p Yet, they may be operated to exhibit very low noise figures and stable phase shift characteristics, parameters comparable to those of more conventional low-power electron beam parametric amplifiers. The beam is effectively supported along its entire length to avoid the creation of potential wells which in turn establish ion traps within the beam, and the positive ions are caused to travel along the electron stream to a point at which they are collected without entering into the electron-wave amplification process.

While a particular embodiment of the present invention has been shown and described, it is apparent that changes and modifications may be made therein without departing from the invention in its broader aspects. The aim lof the appended claims, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. An electron beam device comprising:

a substantially evacuated envelope;

means for projecting an electron stream along a path at a velocity sufficient to ionize residual gas molecules, said stream exhibiting a potential distribution across its width having a positive potential minimum yat its axis tending to attract any positive ions of said gas toward said axis;

means for subjecting said stream to a magnetic field having fiux lines parallel to said path for establishing a condition of transverse electron resonance;

an amplifying assembly having an input coupler responsive to input signal energy for imparting motion to said electrons establishing transverse-mode electron waves on said stream, a pump section downstream from said input coupler responsive to applied energy for developing a periodic inhomogeneous field having a periodicity phased with said motion to increase the magnitude o-f said waves, and an output coupler downstream from said pump section for extracting signal energy from said waves;

a plurality of conductive structures included in said assembly, spaced along said path, and individually spaced about said path to define a drift region and tapering apart in a given direction along said pat-h;

lbias means connected to said conductive structures for establishing, together with the taper of said structures, a negative potential gradient along the portion -of said path from one of said couplers through the other, said gradient being .sufiiciently negative to draw said ions in said direction;

means, spaced along said stream in said direction from said couplers and biased at a potential negative relative to that along said portion, for collecting said positive ions;

and means at the downstream end of said path for collecting said electrons.

2. An electron beam device comprising:

a substantially evacuated envelope;

means for projecting an electron stream along a path at a |velocity sufficient to ioniZe residual gas molecules, said stream exhibiting a potential distribution across its width having a positive potential minimum 10 at its axis tending to attract any positive ions of said gas toward said axis;

means for subjecting said stream to a magnetic field having flux lines parallel to said path for establishing a condition of transverse electron resonance;

an amplifying assembly having an input coupler responsive to input signal energy for imparting motion to said electrons establishing transverse-mode electron waves on said stream, a pump section downstream from said input coupler responsive to applied energy for developing a periodic inhomogeneous field having a periodicity phased with said motion to increase the magnitude of said waves with said pump section including a quadrifilar helix coaxial with said path together with a conduct-ive short connecting all four windings of said helix together at the upstream end thereof, and an output coupler, downstream from said pump section for extracting signal energy from said waves;

a plurality of conductive structures included in said assembly, spaced along said path, and individually spaced about said path to define a drift region and tapering apart in a given direction along said path;

*bias means connected to said conductive structures for establishing, together with the taper of said structures, a negative potential gradient along the portion of said path from one of said couplers to the other, said gradient being sufficiently negative to draw said ions in said direction;

means, spaced along said stream in said direction from said couplers and biased at a potential negative relative lto that along said portion, for collecting said positive ions;

and means at the downstream end of said path for collecting said electrons.

3. An electron beam device comprising:

a substantially evacuated envelope;

means for projecting an electron stream along a path at a velocity sufficient to ionize residual gas molecules, said stream exhibiting a potential distribution across its width having a positive potential minimum at its axis tending to attract any positive ions of said gas toward said axis;

means for subjecting said stream to a magnetic field having flux lines parallel to said path for establishing a condition of transverse electron resonance;

an input coupler, including conductive structure spaced about said path to define a drift region and tapering apart in a given direction along said stream, responsivev to input `signal energy for imparting mot-ion to said electrons establishing transverse-mode electr-on waves on said stream;

a pump section downstream from said input coupler, including conductive structure spaced about said path to define a drift region and tapering apart in :said direction, responsive to applied energy for de- Neloping Ia periodic inhomogeneous field having a periodicity phased with said motion to increase the magnitude of said waves;

an output coupler downstream from said pump section, including conductive structure spaced about said path to define a drift region and tapering apart in said direction, for extracting signal energy from said waves;

bias means connected to different ones of said conductive structures for establishing, together with the taper of said structures, a negative potential gradient along the portion of said path from one of said couplers through the other, said gradient being sufficiently negative to draw said ions in said direction;

means, spaced along said stream in said direction from said couplers and biased at a potential negative relative to that along said portion, for collecting, said positive ions;

and'means at the downstream end ofsaid path for collecting said electrons.

4. An electron beam device comprising:

a substantially evacuated envelope;

means for projecting an electron stream along a path at a velocity suicient to ionize residual gas molecules, said stream exhibiting 'a potential distribution across its width having a positive potential minimum at its axis tending to attract any positive ions of said gas toward said axis;

means for subjecting said stream to 'a magnetic field having ux lines parallel to said path for establishing a condition of transverse electron resonance;

an input coupler, including yconductive structure spaced about said path to define a drift region and tapering apart in a given direction along said 'stream responsive to input signal energy for imparting motion to said electrons establishing transverse-mode electron waves on said stream;

a pump section downstream from said input coupler, including conductive structure spaced about said path to define a drift region and tapering apart in said direction, responsive to applied energy for developing a periodic inhomogeneous held having a periodicity phased with said motion to increase the magnitude of said waves;

an output coupler downstream from said pump section, including `conductive structure spaced about said path to define a drift region and tapering apart in said direction, for extracting signal energy from said waves;

bias means connected to different ones of said conductive structures for establishing, together with the taper of said structures, a negative potential gradient along the portion of said path from one of said couplers through the other, said gradient being suiciently negative to draw said ions in said direction;

means, spaced along said stream in said direction from said couplers and biased at a potential negative relative to that along said portion, for collecting said positive ions;

a conductive element, interposed individually between and substantially encompassing the space between said -couplers and said ion-collecting-means, defining a drift region and tapering apart in said direction;

means for biasing said element at a potential negative relative to that along said path portion;

and means at the downstream end of said path for collecting said electrons.

5. An electron .beam device comprising:

a substantially evacuated envelope;

means for projecting 1an electron stream along a path at a velocity suicient to ionize residual gas molecules, said stream exhibiting a potential distribution across its width having a positive potential minimum at its axis tending to attract any positive ions of said gas toward said axis;

means Afor subjecting said stream to a magnetic iield having fiux lines parallel to said path for establishing a condition of transverse electron resonance;

an input coupler, including conductive structure spaced about said path to `define a drift region and tapering apart in the downstream direction, responsive to input signal energy for imparting motion to said electrons establishing transverse-mode electron waves on said stream; y

a pump section downstream from said input coupler, including conductive structure spaced about said path to dene a drift region and tapering apart in the downstream direction, responsive to applied energy for developing a periodic inhomogeneous field having a periodicity phased with said motion to increase the magnitude of said wave;

'an output coupler downstream from said pump section, including conductive structure spaced about said path to define a drift region and tapering apart in the downstream direction, for extracting signal energy from said waves;

bias means connected to different ones of said conductive structures for establishing, together with the taper of said structures, a negative potential gradient along the portion of said path from said input coupler through said output coupler, said gradient being sufficiently negative to draw said ions downstream;

means, disposed downstream from said output coupler and biased at a potential negative relative to that along said portion, for collecting said positive ions;

and means at the downstream end of said path for collecting said electrons.

6. An electron beam device comprising:

a substantially evacuated envelope;

means for projecting an electron stream along a path at a velocity sufiicient to ionize residual gas molecules, said stream exhibiting a potential distribution across its width having a positive potential minimum at its axis tending to attract any positive ions of said gas toward said axis;

means for subjecting said stream to a magnetic field having Iiiux lines parallel to said path `for establishing a condition of transverse electron resonance;

an input coupler, including conductive structure spaced about said path to define a drift region and tapering apart in the downstream direction, responsive to input signal energy for imparting motion to said electrons establishing transverse-mode electron waves on said stream;

a pump section downstream from said input coupler, including conductive structure spaced about said path to define a drift region and tapering apart in the downstream direction, responsive to applied energy for developing a periodic inhomogeneous held having a periodicity phased with said motion to increase the magnitude of said waves;

an output coupler downstream from said pump section, including conductive structure spaced about said path to define a drift region and tapering apart in the downstream direction, for extracting signal energy from said waves;

bias means connected to different ones of said conductive structures for establishing, together with the taper of said structures, a negative potential gradient along the portion of said path from said input coupler through said output coupler, said gradient being suflciently negative to draw said ions downstream;

means, spaced along one side of -said stream downstream from said output coupler and lbiased at a potential negative relative to that along said portion, for collecting said positive ions;

a conductive element, interposed individually ybetween and substantially encompassing the space between said output coupler and said ion-colle'cting-means, defining a drift region and tapering apart in the downstream direction;

means for biasing said element at a potential negative relative to that along said portion;

and means, disposed along said path and opposite said ion-collecting-means, biased to a positive potential for collecting said electrons.

7. An electron beam device comprising:

a substantially evacuated envelope;

means yfor projecting an electron stream along a path at a velocity suiiicient to ionize residual gas molecules, said stream exhibiting a potential `distribution across its width having a positive potential minimum at its axis tending to attract any positive ions of said gas toward said center;

means for subjecting said stream to amagnetic field having fiux lines parallel to said path for establishing a condition of transverse electron resonance;

an input coupler responsive to input signal energy for imparting motion to said electrons establishing transverse-mode electron waves on said stream;

a pump section downstream from said input coupler responsive to applied energy for `developing a periodic nhomogeneous field having a periodicity phased with said motion to increase the magnitude of said waves;

an output coupler downstream from said pump section -for extracting signal energy from said waves;

means interposed individually between and substantially encompassingthe space between each lof said couplers and said pump section, including conductive structures spaced about said path to define a drift region and tapering apart in a given direction along said stream;

bias means connected to different ones of said conductive structures, couplers and pump section for establishing, together with the taper of said structures, a negative potential gradient along the portion of said path from one of said couplers through the other, said gradient being sufficiently negative to draw said ions in said direction;

means, spaced along said stream in said direction from said couplers and biased at a potential negative relative to that along :said portion, for collecting said positive ions;

and means at the downstream end of said path for collecting said electrons.

8. An electron beam device comprising:

a substantially evacuated envelope;

means for projecting an electron stream along a path at a velocity suflicient t-o ionize residual gas molecules, said stream exhibiting a potential distribution across its width having a positive potential minimum at its axis tending to attract any positive ions of said gas toward said axis;

means for subjecting said stream to a magnetic field having flux lines parallel to said path for establishing a condition of transverse electron resonance;

an input coupler responsive to input signal energy for imparting motion to said electrons establishing transverse-mode electron waves on said stream;

a pump section downstream from said input coupler responsive to applied energy -for developing a periodic inhomogeneous field having a periodicity phased with said motion to increase the magnitude of said waves;

an output coupler downstream from said pump section for extracting signal energy from said waves;

means interposed individually between and substantially encompassing the space between each of said couplers and said pump section, including conductive structure spaced about said path to define a drift region and tapering apart in the downstream direction;

bias means connected to different ones of said conductive structures, couplers and pump section for establishing, together with the taper of said structures, a negative potential gradient along the portion o-f said path from said input coupler through said -output coupler, said gradient being sufficiently negative to draw said ions downstream;

means, disposed downstream from said output coupler and biased at a potential negative relative to that along said portion, for collecting said positive ions;

and means at the downstream end of said path for collecting said electrons.

9, An electron beam device comprising:

a substantially evacuated envelope;

means for projecting an electron stream along a path at a velocity s-uflicient to ionize residual gas mo-lecules, said stream exhibiting a potential distribution across its width having a positive potential minimum at its axis tending to attract any positive ions of said gas toward said axis;

means for subjecting said stream to a magnetic field having flux lines parallel to said path for establishing la condition of transverse electron resonance;

an input coupler, including conductive structure spaced about said path to define a drift region and tapering apart in a given direction `along said stream responsive to input signal energy for imparting motion to said electrons establishing transverse-mode electron waves on said stream;

a pump section downstream from said input coupler, including conductive strcture spaced about said path to define a drift region andtapering apart in said direction, responsive to applied energy for developing a periodic inhomogeneous field having a periodicity phased with said motion to increase the magnitude of said waves;

an output coupler downstream from said pump section, including conductive structure spaced about said path to define a drift region and tapering apart in said direction, for extracting .signal energy from said waves;

means interposed individually between and substantially encompassing the space between each of said couplers and said pump section, including conductive structure spaced about said path to define a drift region and tapering apart in said direction;

bias -means connected to different ones of said conductive structures for establishing, together with the taper of said structures, a negative potential gradient along the portion of said path from one of said couplers through the other, said gradient being sufficiently negative to draw said ions in said direction;

means, spaced along said stream in said direction from said couplers and biased `at a potential substantially negative relative to that along said portion, for collecting said positive ions;

a conductive element, interposed individually between and substantially encompassing the space between said couplers and said ion-collecting-means, defining a drift region and tapering `apart in said direction;

means for biasing said element at a potential substantially negative relative to that along said portion;

and lmeans at the downstream end of said path for collecting said electrons.

10. An electron beam device comprising:

a substantially evacuated envelope;

means for projecting an electron stream along a path at a velocity sufiicient to ionize residual gas molecules, said stream exhibiting a potential distribution across its width havin-g a positive potential minimum at its axis tending to attract any positive ions of said gas toward said axis;

means for subjecting said stream to a magnetic field having flux lines parallel to said path for establishing a condition of transverse electron resonance;

an input coupler, including conductive structure spaced about said path to define a drift region and tapering apart in the downstream direction, responsive to input signal energy for imparting motion to said electrons establishing transverse-mode electron waves on said stream;

a pump section downstream yfrom said input coupler, including conductive structure spaced about said path to define a drift region and tapering apart in the downstream direction, responsive to applied energy for developing a periodic inhomogeneous field having a periodicity phased with said motion t-o increase the magnitude of said waves;

an output coupler downstream from said pump section, including conductive structure spaced about said path lto define a drift region and tapering apart in the downstream direction, for extracting signal energy from said waves;

means interposed individually between and substantially encompassing the space between each of said cou- 15 16 plers and said pump section, including conductive a conductive element, interposed individually between structure spaced about said path to dene a drift and substantially encompassing the space between region and tapering apart in the downstream direcsaid output coupler and saidion-collecting-means, detion; ining a drift region and tapering apart in the downbias means connected to different ones of said con- 5 stream direction;

ductive structures for establishing, together with the means for biasing said element at a potential substantaper of said structures, a negative potential gradient tially negative relative to that along said portion; along the portion of said path from said input couand means at the downstream end of said path for colpler through said output coupler, said gradient being lecting said electrons. sufficiently negative to draw said ions downstream; 10 means, disposed downstream from said output coupler No references cited.

and biased at a potential substantially negative relaa tive to that along said portion, for collecting said ROY LAKEPrlmWJEXamme- Positive ions? D. R. HOSTETTER, Assistant Examiner. 

1. AN ELECTRON BEAM DEVICE COMPRISING: A SUBSTANTIALLY EVACUATED ENVELOPE; MEANS FOR PROJECTING AN ELECTRON STREAM ALONG A PATH AT A VELOCITY SUFFICIENT TO IONIZE RESIDUAL GAS MOLECULES, SAID STREAM EXHIBITING A POTENTIAL DISTRIBUTION ACROSS ITS WIDTH HAVING A POSITIVE POTENTIAL MINIMUM AT ITS AXIS TENDING TO ATTRACT ANY POSITIVE IONS OF SAID GAS TOWARD SAID AXIS; MEANS FOR SUBJECTING SAID STREAM TO A MAGNETIC FIELD HAVING FLUX LINES PARALLEL TO SAID PATH FOR ESTABLISHING A CONDITION OF TRANSVERSE ELECTRON RESONANCE; AN AMPLIFYING ASSEMBLY HAVING AN INPUT COUPLER RESPONSIVE TO INPUT SIGNAL ENERGY FOR IMPARTING MOTION TO SAID ELECTRONS ESTABLISHING TRANSVERSE-MODE ELECTRON WAVES ON SAID STREAM, A PUMP SECTION DOWNSTREAM FROM SAID INPUT COUPLER RESPONSIVE TO APPLIED ENERGY FOR DEVELOPING A PERIODIC INHOMOGENEOUS FIELD HAVING A PERIODICITY PHASED WITH SAID MOTION TO INCREASE THE MAGNITUDE OF SAID WAVES, AND AN OUTPUT COUPLER DOWNSTREAM FROM SAID PUMP SECTION FOR EXTRACTING SIGNAL ENERGY FROM SAID WAVES; 